Carbon Oxides





The carbon oxides are stable, diamagnetic molecules. There is a C-O triple bond in CO with formal charges on each atom.

Carbon dioxide has a formal double bond between C-O.

The Lewis structure of carbon monoxide shows that it is relatively electron-rich at carbon. Carbon dioxide is electron-poor at the central carbon and acts as an electrophile.

The molecular orbital diagram of carbon monoxide is very similar to that of molecular nitrogen. Carbon, with 4 valence electrons, and oxygen with 6 valence electrons, together have the same number of electrons as dinitrogen.

Oxygen is more electronegative than carbon, so its orbitals are more highly stabilized and lower in energy than the carbon orbitals.

Both of these molecules have 3 filled bonding orbitals. There are also two orbitals that, together, are nonbonding. One of the non-bonding pairs is closer in energy to the carbon atomic orbitals and the other is closer in energy to the oxygen atomic orbitals.
In carbon dioxide, the carbon 2s and 2p(z) orbitals combine with 2 combination orbitals (O2s+O2s and O2s-O2s) to make 2 sigma bonding and 2 sigma antibonding orbitals. The carbon 2p(x) combines with the group orbital O2p(x)+O2p(x) and the carbon 2p(y) combines with the group orbital O2p(y)+O2p(y). This makes 2 pi bonding molecular orbitals and 2 pi antibonding molecular orbitals. The remaining 4 p orbitals on the oxygen atoms are non-bonding molecular orbitals.

It is important to note that the sigma and the pi orbitals extend over the whole 3-atom molecule.

Combustion

Combustion of any organic molecule in a minimum of oxygen produces carbon monoxide. Below is the balanced reaction for the combustion of isobutane.



The reaction is exothermic. You can calculate the enthalpy of reaction by subtracting the enthalphies of formation of products from reactants. The enthalpy of formation of any element in its most common form is zero.

Hrxn = {(10 mol)(-462.75 kJ/mol) + (12 mol)(-1012.49 kJ/mol)}-{(2 mol)(-154.49)} = 1.7 104 kJ or 1.7 103 kJ/C

In an excess of oxygen, carbon dioxide is produced along with more energy.



Hrxn = {(5 mol)(-1647.54 kJ/mol) + (6 mol)(-1012.49 kJ/mol)}-{(1 mol)(-154.49)} = 1.4 104 kJ or 2.8 103 kJ/C

The combustion of acetylene is also exothermic.



None of these reaction schemes gives a true picture of combustion in a flame. It is much more complex than this. Mechanistic studies show multiple steps that product both larger and smaller hydrocarbon and oxyhydrocarbon species before the ultimate formation of water and carbon dioxide. Soot, Cs, is also a product.

Below is a simplified scheme that shows the principle reactions in the combustion of acetylene, C2H2, in a flame. The ultimate products are CO2 and H2O.

Carbon Monoxide Properties

Carbon monoxide is flammable and it is very toxic and odorless. It binds strongly with hemoglobin, the oxygen-carrying protein in blood and prevents oxygen binding. It dissolves in water without reaction.
  • Molecular weight = 28.01 g/mol

  • Boiling point = -191.6 deg C

  • Specific gravity (air = 1) (1.013 bar and 21 deg C (70 deg F)) = 0.968
  • Solubility in water (1.013 bar and 0 deg C (32 deg F)) = 0.0352 vol/vol

Carbon Dioxide Properties

Carbon dioxide is produced from combustion, fermentation, and is a product of respiration in animals. It is assimilated by plants in photosynthesis. CO2 gas has a slightly irritating odor, is colorless and heavier than air. It freezes at -78.5 deg C at normal atmospheric pressure to form "dry ice" without going through a liquid phase. It reacts with water to form carbonic acid.
  • Molecular weight = 44.01 g/mol

  • Boiling point (Sublimation) = -78.5 deg C

  • Specific gravity (air = 1) (1.013 bar and 21 deg C (70 deg F)) = 1.521
  • Solubility in water (1.013 bar and 0 deg C (32 deg F)) = 1.7163 vol/vol

Carbon Monoxide Reactivity

We have seen that the carbon of CO is electron-rich. It reacts with electrophiles. In the reactions below, the CO molecule complexes to a nickel center. It also reacts with hydroxyl radical. All radical species are electron-poor.



Carbon Dioxide Reactivity

The carbon in CO2 is electron-poor and it reacts with nucleophiles. In the reactions below, the nucleophiles water and hydroxide react at the carbon.




Professor Patricia Shapley, University of Illinois, 2010